This invention relates to a thermometer which is capable of measuring cryogenic temperatures with high accuracy and is serviceable over a wide temperature range.
Secondary thermometers used for measuring cryogenic temperatures come in two types, i.e. resistance thermometers and magnetic thermometers. The resistance thermometers are designed to effect measurement of temperatures by virtue of the variation in resistance which a metal such as, for example, platinum, nickel or copper, manifests in response to the change in temperature. They operate with comparatively satisfactory repeatability so far as the measurement is limited to a temperature range above 20 mK, but cannot be adapted to the measurement in a wide temperature range below 100 mK at which accurate temperature measurement is required today in the fields of industrial science and technology. Magnetic thermometers effect measurement of temperatures by making use of the fact that magnetic susceptibility is inversely proportional to temperature. They are useful for the measurement of temperatures in a range below 1.degree. K. When transported however, their device constants are inevitably affected. Thus, they entail a serious disadvantage that they are not capable of providing reliable means of international comparison and standardization.
A normal conductor in the proximity of a superconductor causes repulsion of magnetic field, a phenomenon called the Meissner effect. The volume of the repulsion of magnetic field due to this phenomenon varies as a function of temperature. Attempts have been made to use the Meissner effect for the measurement of temperature by determining temperature from the measured magnetic permeability of the ordinary conductor. The thermometer embodying this idea, therefore, must use a large coil for the measurement of the magnetic permeability and, therefore, necessitates incorporation of a large housing for accommodating the coil. Besides, the measured temperature obtained by this thermometer is affected by device constants such as of the aforementioned coil and also by the repeating cycles of the change of temperature between normal room temperature and the temperature to be measured. Owing to these disadvantages, this thermometer has so far failed to find acceptance as a device for international comparison and standardization. Further, this thermometer can accurately measure temperature only over a very narrow temperature range and it has been difficult to provide accurate temperature measurement over a wide temperature range with one thermometer.
Under the circumstances a need has been felt for a small, light and easily portable secondary thermometer capable of providing highly accurate temperature measurement over a wide temperature range below 100 mK.